The Growth and Characterization of InN Quantum Dots Using Droplets Epitaxy in MBE

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III-nitride semiconductor material systems offer great potential for next-generation optoelectronic devices due to their direct bandgaps, which vary from 0.7 eV (InN) to 3.5 eV (GaN) to 6.2 eV (AlN), as well as their other unique properties. InN has gained much less attention than GaN and AlN within this family of semiconductors due to its complicated low-temperature growth. However, the prediction that an InN quantum well on GaN can become a two-dimensional (2D) topological insulator has resulted in expanding the research interest in InN. At the same time, this renewed interest has begun to reveal that the formation of an appropriate 2D InN film is difficult at best and physically forbidden by strain at worst. This has shifted the focus on InN to 3D nanostructures in an attempt to achieve similar novel affects. However, this shifted focus has revealed a challenging landscape for the study of the growth of these 3D nanostructures. This research has focused on investigating the growth of InN quantum dots (QDs) by droplet epitaxy (DE) using radio frequency plasma-assisted molecular beam epitaxy (MBE) in order to discover and learn to control the growth kinetics of this novel system. The QD growth kinetics from the formation of liquid In droplets to the crystallization of InN QDs was studied with a focus on the effects of ambient nitrogen and substrate type and temperature. The substrates studied were c-plane sapphire and (0001) GaN, while the temperature varied from nearly room temperature to ~400 °C. The growth quality, dot density, diameter, and height of In droplets as well as InN QDs were investigated utilizing reflection high-energy electron diffraction (RHEED), X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM), and Scanning Electron Microscopy (SEM). The droplet formation was determined to follow well known principles of nucleation theory with ripening. By analyzing the areal density of nanostructures as functions of temperature, the corresponding activation energies for surface diffusion where determined. The resulting activation energy for In surface diffusion on sapphire was found to be 0.62 ± 0.07 eV in ultra-high vacuum, ~10-10 Torr, and 0.57 ± 0.08 eV in ambient N2, ~10-5 Torr. For the InN QDs on GaN, the resulting activation energy for In surface diffusion on GaN was found to be 0.23 ± 0.03 eV. In addition, it was found that by analyzing the density of crystallized QDs, following the droplet formation under ambient N2, a very close activation energy of 0.25 ± 0.1 eV was found.
MBE, Quantum Dots, InN, Droplets Epitaxy